In the compressor and turbine sections of gas turbine engines, blades rotate about the axis of the engine. The blade tips come in proximity to the inner wall of the engine case, frequently rubbing the case wall, or an abradable seal or rubstrip located on the case wall. Engine efficiency depends to a significant extent upon minimizing leakage by control of the gas flow in such a manner as to maximize interaction between the gas stream and the moving and stationary blades. A major source of inefficiency is leakage of gas around the tips of the compressor blades, between the blade tips and the engine case or seal means. In the highly competitive gas turbine business, there is a strong emphasis on providing closer tolerances as a means to achieve greater efficiencies. Although a close tolerance fit may be obtained by fabricating the mating parts to a very close tolerance range, such a fabrication process is extremely costly and time consuming. Further, when the mated assembly is exposed to a high temperature environment and high stress, as when in use, the coefficients of expansion of the mating parts may differ, thus causing the clearance space to either increase or decrease. The latter condition would result in a frictional contact between the blades and the housing, causing elevation of temperatures and possible damage to one or both members. On the other hand, increased clearance space would permit gas to escape between the blade and housing, thus decreasing efficiency.
One means to increase efficiency is to apply a coating of suitable material to the interior surface of the housing, to reduce leakage between the blade tips and the housing. Various coating techniques have been employed to coat the inside diameter of the housing with an abradable coating which can be worn away by the frictional contact of the rotating blade, to provide a close fitting channel in which the blade tip may travel. Thus, when subjecting the coated turbine assembly to a high temperature and stress environment, the blade and the case may expand or contract without resulting in significant gas leakage between the blade tip and the turbine housing. This abradable coating technique has been employed to not only increase efficiency, but to also provide a relatively speedy and inexpensive method for restoring excessively worn turbine engine parts to service.
To extend the life of the blade tips which rub against the abradable seals, abrasive layers are sometimes applied to the blade tip surface by a variety of methods. See, for example, U.S. Pat. No. 4,802,828, of Rutz et al, which suggests several techniques for providing an abrasive layer on a blade tip, including powder metallurgy techniques, plasma spray techniques, and electroplating techniques; Schaefer et al, U.S. Pat. No. 4,735,656, which teaches application of an abrasive comprising ceramic particulates in a metal matrix by controlled melting and solidification of the matrix metal; or, Schaefer et al, U.S. Pat. No. 4,851,188, which teaches a sintering operation for application of an abrasive layer to the tip of a superalloy gas turbine blade.
Electroplating techniques have been previously used for the deposition of abrasive layers to blade tips, as illustrated in Routsis, et al, U.S. Pat. No. 5,074,970, which teaches entrapment of nonconductive particulates within a layer of nickel upon the surface of a compressor blade by electroplating nickel onto a titanium airfoil, submerging the blade tip in a slurry of the particulate in a nickel plating solution, and electroplating a layer of nickel about the particulates in contact with the blade tip surface to encapsulate them in place. Similarly, U.S. Pat. No. 4,608,128, of Farmer et al, relates to deposition of nonconductive particulates on a substrate by applying to the blade tip a nonconductive tape carrying the particles, and electrodeposition of a metallic coating through pores in the tape onto the blade surface and about the abrasive particles, followed by removal of the tape so as to leave the particles on the blade surface, held in place by the electrodeposited metallic coating.
In Stalker et al, U.S. Pat. No. 4,169,020, an abrasive tip is produced by electrodepositing the metal matrix while concurrently entrapping abrasive particles included in the electroplating solution. In this reference, particles are deliberately left protruding from the matrix by limiting the matrix thickness. Wride et al, in U.S. Pat. No. 5,076,897, teach application of a binding coat on the tip of a blade body by electrodeposition, followed by composite electrodeposition of particulate abrasive and an anchoring metal matrix, followed by plating an infill around the abrasive particles.
However, it has been found that in normal use, abrasively tipped compressor blades show wear, erosion, and actual breakage, often referred to as notching, at the points where the blade tip surface intersects the leading edge and the trailing edge of the blade. This portion of the compressor blade, which is normally quite thin, is subject to wear and breakage where said edges contact the abradable coating or rubstrip portion of the gas turbine seal. This has been particularly noted in seal systems which employ relatively hard, smooth, non-porous abradable coatings, such as those comprising plasma sprayed oxidation resistant metal matrix seals containing a lubricating amount of hexagonal boron nitride particulate, or Filled Feltmetal (FFM) abradable rubstrips.
Accordingly, it is an object of the present invention to provide an improved abrasive compressor or turbine blade tip which contributes to engine efficiency by withstanding wear and erosion, and decreasing air flow around the leading and trailing edges. It is a further object of this invention to provide a compressor or turbine blade tip having extended life and reliability due to its resistance to notching. It is also an object of the present invention to provide an improved abradable seal/abrasive blade tip combination, which contributes to engine efficiency, reliability, and durability, by providing a compressor seal, which while abradable and smooth, is impermeable to gas flow, in combination with a highly abrasive blade tip which is itself resistant to erosion and wear.